Add a basic performance test to the selenium suite.

@fmarier, @lloyd, @kparlante, @jedp - this could use some review!

Releasing this to the wild to get review on the approach.

* Must be manually turned on for it to run.
* Requires an update to kpiggybank to be able to fetch the "last" result - https://github.com/mozilla/kpiggybank/pull/18
* Adds a new KPI that should probably go into the event stream - ready_time_ms. This is the amount of time it takes for the communication_iframe to load.